There are only a limited number of ways to add solder to relatively small areas, such as pads on flexible circuits or pads on other types of substrates. One commonly used technique is to screen solder paste onto the contacts formed on a substrate in a conventional screening process. Thereafter, preformed solder balls are adhered to the wet paste using a thin plate having openings therein to locate the solder balls. Thereafter, the thin plate is removed and the solder reflowed to form the solder ball connection to the pad.
Another technique is to use preformed solder balls and a ball holder, and place the substrate having the ball grid array (BGA) pads onto the solder balls. This assembly is then placed in an oven having a suitable atmosphere, such as a hydrogen gas, which can act as a flux and reflowed to join the balls to the pads.
Another technique is to place all of the solder balls in a block, place the substrate over the solder balls and apply short bursts of high current to the ball pads thus melting some solder on the solder balls, thus causing each solder ball to adhere to its respective pad.
While all of these techniques are reasonably successful and provide good solder connections, nevertheless, all of these techniques involve a high degree of handling of multiple parts, including handling of individual solder balls, and involves a batch operation as opposed to a continuous operation. In high volume situations, there needs to be very close process control, and often optical arrangements for monitoring the process. In any event, in high volume, repetitive situations, the batch process together with individual handling of the pieces contributes to a relatively expensive process.
It is therefore ore a principal object of the present invention to provide an improved technique for continuously applying solder balls to a substrate such as a flexible circuit or other type of substrate wherein there are pads provided in a predetermined pattern in which the solder balls are applied.